Fróðskaparrit - 01.01.1996, Síða 123

IMAGING OF BASALT AND UNDERLYING STRUCTURES IN THE FAROESE OFFSHORE AREA 127
1. The basalt seems to subdivide into two units, the
light brown Basalt and yellow-brown ?Basalt. The
division between these two units has been based
upon the apparent thickening of the light brown unit
in the region of wd 755 and thinning near wd 784;
this thinned area appears to have been uplifted and
eroded, as the surface is rough rather than smooth
and the substantially plane parallel bedded character
prevalent between wd 739 and wd 786 has been
removed.The underlying ?Basalt is more constant in
thickness but still retains a plane parallel nature. On-
lap and downlap terminations with the basal brown
unit are absent.
2. The brown Subbasalt A unit displays a strong reflec-
tion boundary with the overlying ?Basalt unit. The
brown series displays bedded to massive character
and does infill its basal Subbasalt B light yellow
layer. It also thins over the high in jhe regon of wd
810.
3. Subbasalt B is a very thick, again massive to bedded
unit, with very strong basal reflectors. Little intemal
structure is seen but the unit again thickens over the
high.
4. The deep yellow Subbasalt 4 displays the strong
reflection boundary typical of this unit, some internal
form characteristics, and strong block faulting.
These faults seem to control the uplift which has
created the high over which the overlying units thins.
The relative presence or absence of thinning most
probably represents reactivation of existing trends
over time. The basin itself displays indications of
subsidence, typical of an area which is being loaded
and downwarped prior to the continent-ocean transi-
tion.
Discussion
Upon examination and mapping of the seis-
mic data, it became evident that the Faroese
offshore area could be divided into two dif-
ferent geophysical environments: those re-
gions where it was possible to seismically
image within and beneath the basalts, and
those areas where imaging was not current-
ly possible.
Broadly speaking, in the shallower water
areas, where there is little Tertiary sediment
cover and where the basalt itself either out-
crops, or approaches outcrop, of the sea
floor, it has not proven possible to either
discretely penetrate the basalts or to coher-
ently discriminate the recorded signals
from recorded noise and artefacts. These
artefacts might be either geophysical acqui-
sition system related (source overshooting;
multiple generation; poor signal to noise
ratio; source/receiver pattern effects, etc.)
or geologically related (geologically in-
duced ringing; Q factors of the various rock
units; earth induced multiple generation;
scattering of incident wave energy by dis-
rupted surfaces, etc.); in any case, the cause
of this phenomena is unclear. The end re-
sult is that discrete reflector sequences can-
not be determined either within or below
the basalt in these areas, which roughly cor-
respond to the 300 m bathymetric contour
along the Faroese Platform area.
However, in the more basinal environ-
ments where water depths are deeper and
the amount of Tertiary sedimentation in-
creases, imaging within and below the
basalt can be accomplished as reflected
seismic energy is both transmitted and re-
ceived discretely. This allows observation
of coherent reflector packages throughout
the area which can be regionally identified
and correlated.
Imaging capability also appears to be re-
lated to thickness of the basalts within these
basinal areas. Where the basalt »icing« is
thin, or the subaerial flows are interfingered
with sedimentary units, it is possible to de-
fine with a certain degree of confidence the
underlying basin form itself and the margin
faults which control it. The converse argu-